1. Field of the Invention
The invention relates to an internal combustion engine.
2. Description of Related Art
In internal combustion engines, air-fuel ratio control is performed that involves correcting a fuel injection amount in such a manner that an actual air-fuel ratio becomes a target air-fuel ratio. In an internal combustion engine provided with a plurality of cylinders, as disclosed in Japanese Patent Application Publication No. 2012-233425 (JP 2012-233425 A), variation in the air-fuel ratio among cylinders occurs, and emissions become worse, when a fuel injection amount varies among the cylinders; accordingly, there is determined the presence or absence of a variation in the air-fuel ratio among the cylinders.
Internal combustion engines that are provided with an intake port fuel injection valve that injects fuel to an intake port, and with an in-cylinder fuel injection valve that injects fuel directly into a fuel combustion chamber, are likewise available. In such internal combustion engines, as conventionally available, the performance of the internal combustion engine is enhanced through execution of split injection of fuel, by modifying, in accordance with an engine operating state, an injection proportion of fuel that is injected through each fuel injection valve. For instance, the performance of the internal combustion engine, in terms of suppressing oil dilution by fuel and suppressing the formation of soot in the fuel combustion chamber, is enhanced by increasing the proportion of fuel that is injected through the intake port fuel injection valve, and by reducing the proportion of fuel that is injected through the in-cylinder fuel injection valve. By conversely reducing the proportion of fuel that is injected through the intake port fuel injection valve and increasing the proportion of fuel that is injected through the in-cylinder fuel injection valve, the amount of fuel that vaporizes inside the fuel combustion chamber increases, and the intake temperature drops as a result. The performance of the internal combustion engine is accordingly enhanced in that, for instance, anti-knocking performance and filling efficiency of the intake are enhanced as a result.
For instance, JP 2012-233425 A discloses a feature wherein variation in the air-fuel ratio among cylinders occurs in such an internal combustion engine, there is determined whether this variation is caused by variation in the amount of fuel that is injected through each intake port fuel injection valve, or variation in the amount of fuel that is injected through each in-cylinder fuel injection valve. Further, JP 2012-233425 A discloses the feature of prohibiting the use of the fuel injection valve that is the cause of variation in the air-fuel ratio, to curtail thereby worsening of emissions derived from variation in the air-fuel ratio among cylinders.
An optimal value in terms of enhancing the performance of the internal combustion engine is set herein as the injection proportion at the time of execution of the above-described split injection. When prohibiting the use of the fuel injection valve due to which the fuel injection amount varies among cylinders, therefore, it becomes possible to suppress worsening of emissions derived from variation in the air-fuel ratio, which is caused in turn by variation in the fuel injection amount, however, the injection proportion of fuel can no longer be optimized. Accordingly, the effect of enhancing the performance of the internal combustion engine through split injection of fuel, such as the above-described one, is impaired.